The present invention generally relates to noise reduction circuits for video signal, and more particularly to a noise reduction circuit which can effectively eliminate a noise component existing immediately after a rise in a luminance signal of a reproduced video signal.
Generally, a noise component is mixed in a video signal which is reproduced from a magnetic tape in a video signal magnetic recording and reproducing apparatus. However, in a recording system of the magnetic recording and reproducing apparatus, especially a high frequency component of a luminance signal of the video signal which is recorded and reproduced is subjected to a pre-emphasis and then to a frequency modulation. For this reason, a frequency of a frequency modulated signal especially at a part immediately after a part (rise) where the luminance signal rapidly changes from a black level to a white level and at a part immediately after a part (fall) where the luminance signal rapidly changes from a white level to a black level is high because of the pre-emphasis performed in the recording system. But in a general magnetic recording and reproducing apparatus, a signal-to-noise (S/N) ratio of a recording and reproducing characteristic is poor in a high frequency range. Hence, the S/N ratio is poor at the parts immediately after the rise and immediately after the fall in the luminance signal, and it is desirable to positively eliminate the noise component existing at such parts of the luminance signal.
Accordingly, a noise reduction circuit for video signal was proposed in a U.S. patent application Ser. No. 389,041 filed June 16, 1982, now U.S. Pat. No. 4,563,704 in which the assignee is the same as the assignee of the present application. This previously proposed noise reduction circuit comprises a first circuit which can eliminate the noise component existing immediately after the rise in the luminance signal waveform and a second circuit which can eliminate the noise component existing immediately before the fall in the luminance signal waveform, and the first and second circuits are coupled in series.